1. Field of the Invention
The present invention relates to a radiographic apparatus and a control method therefor, and more particularly, to a control method based on pressure information fluctuations applied to a radiation sensor portion as an image detection unit.
Note that, description herein is made taking X-rays as an example of the radiation, but the present invention is not limited to X-rays, and electromagnetic waves such as γ-rays and α-rays and β-rays are also included in the radiation.
2. Description of the Related Art
An X-ray sensor in which a semiconductor is formed on a glass substrate is used in taking a still image and a moving image. Both when a still image is taken and when a moving image is taken, offset correction is made between offset images before and after X-ray exposure. However, the offset correction is made on the understanding that the offset image is stable during the image taking period.
The inventors of the present invention have found that, when an X-ray sensor is inserted under the body of a patient, body motion of the patient causes subtle changes (fluctuations) in the offset image. The offset image fluctuations are assumed to be due to pressure fluctuations applied to the X-ray sensor, and also, due to slimming down of the glass substrate on which the semiconductor is formed.
Japanese Patent Application Laid-Open No. 2009-178196 discloses a technology of using an X-ray sensor in conjunction with a pressure sensor. Specifically, a contact state is detected from the pressure sensed by the pressure sensor when a photographed body part of a patient is set in an electronic cassette, whether or not the pressure sensor within the range of a specified part is in a contact state is determined, and the result of the determination is reported to a radiographer via a display and a speaker.
Japanese Patent Application Laid-Open No. 2009-172250 discloses a technology of, when the absolute value of the difference between the area of a set contact range between an X-ray sensor and a subject and the area of the actual contact range is larger than an allowable threshold value of the area, determining that the photographed body part of a patient is not set in the electronic cassette in a desired state, and reporting the determination to a radiographer.
On the other hand, Japanese Patent Application Laid-Open No. 2003-190126 discloses a technology relating to a method of collecting offset images of an X-ray sensor. Specifically, first, prior to taking a radiation image, the rate of change is calculated of an output signal of an integrator in a predetermined time period from a reset of the integrator to a supply of an exposure start signal. This rate of change is used to calculate an offset voltage signal at an arbitrary time, which is supplied to a voltage correction circuit. By correcting an output signal of the integrator after the exposure starts with the offset voltage signal and then supplying the corrected output signal to an X-ray control portion, exposure of the applied X-rays is controlled.
Further, in Japanese Patent Application Laid-Open No. 2006-158728, radiation is applied from a radiation emitting portion to a subject with a predetermined period. A subject image based on the applied radiation is detected by a photoelectric conversion circuit, and an offset image is periodically acquired. In response to a change in the acquired periodical offset image, the radiation application period of the radiation emitting portion and the period of reading a subject image from the photoelectric conversion circuit are controlled. Immediately after the start of photographing where offset fluctuations occur, offset photographing and subject photographing are alternately performed so as to correctly make offset correction of the subject image. When the offset becomes stable, subject photographing may be continuously performed at a high frame rate.
An X-ray sensor in which a semiconductor is formed on a glass substrate is used in, for example, portable roentgenographic apparatus, and the X-ray sensor is inserted under the body of a patient and an image is taken. As described above, the inventors of the present invention have found that, when the X-ray sensor is inserted under the body of a patient, body motion of the patient causes subtle changes (fluctuations) in the offset image. The offset image fluctuations cause an error in the offset correction, which degrades the quality of the diagnostic image. Fluctuations of an offset image are assumed to be due to pressure fluctuations applied to the X-ray sensor, but the actual cause has not been identified yet. Body motion of a patient is broken down into single body motion and periodical body motion. Periodical body motion includes heartbeat and respiratory body motion. The periods of the heartbeat and respiratory body motion are 0.5 seconds to 3 seconds.